What people use skype for:
* chat (many alternatives)
* video calls (many alternatives)
* PSTN gateway (SIP is an alternative. It is handy sometimes, but loosing relevance as IP connectivity is becoming more global)
* conferencing (no good known alternative, which is as simple, ad-hoc)
* screen sharing (many alternatives, but it is ad-hoc, simple.)
* simple file sharing (many alternatives)
Now screen sharing and ad-hoc conferences are very powerful combo, and I'm looking for alternatives, as I'm afraid skype will be totally useless for me in a year or so, if the current trends hold on.